1. Technical Field
The present invention generally relates to wireless communication networks, and particularly relates to a method and apparatus for linearly preceding downlink transmissions to reduce temporal variations in interference.
2. Background
Linear preceding, where different weighting vectors are applied to different signal components to control their transmission from multiple transmit antennas, has been shown capable of improving downlink throughput in wireless communication networks. The improvements, however, largely depend on the availability of instantaneous channel quality information with respect to the receiving mobile stations. Instantaneous channel quality information allows the supporting network transmitter(s) to adapt its (theirs) transmission to a given mobile station based on currently prevailing interference conditions at the mobile station.
In practice, however, network transmitters do not operate with instantaneous channel knowledge because there always is some delay between the time that mobile stations measure interference and report channel quality, and the time that a transmitter adapts a given mobile station's transmit link in response to the last reported channel quality. That delay means that the transmitter's link adaptations lag actual reception conditions at the mobile station, meaning its link adaptations are inappropriate to the extent that reception conditions at the mobile station change over the lag.
The particular linear precoding settings in use at a given base station, i.e., which Orthogonal Frequency Division Multiplex (OFDM) subcarriers are precoded with which weight sets, strongly influence the characteristics of interference caused by the base station's transmissions at nearby mobile stations. Thus, to the extent that these precoding settings change rapidly, the interference conditions at individual mobile stations also change rapidly.